In fuel cell vehicles, under driving conditions in which cooling performance is reduced, for example, when a fuel cell stack is operated with high output, e.g., during a high-temperature climbing driving process, the driving temperature of the fuel cell stack is increased, and the humidity of supply fuel is reduced. Thereby, the fuel cell stack is dried, so that the driving voltage of the stack is reduced under the same current conditions. In this case, a vicious cycle may be caused, in which a heat generation rate of the fuel cell stack is increased by a stack voltage drop, and therefore the driving temperature of the fuel cell is further increased.
To prevent such a vicious cycle of an increase of the fuel cell driving temperature, recent fuel cell systems for vehicles are employing a control technique of increasing the pressure of air to be supplied to a cathode and increasing relative humidity of the cathode side. Given this, there is a need for further increasing a compression ratio of an air compressor which supplies air to the cathode side of the fuel cell stack.
Due to the necessity of further increasing the compression ratio of air to be supplied to the cathode side of the fuel cell stack, an air compressor has been designed such that the compression ratio of the air compressor is increased, and the maximum efficiency is obtained at the maximum pressure driving point. However, although this design can increase efficiency of the compressor in a high flow rate and high compression ratio period, the efficiency of the compressor is comparatively reduced in a low flow rate period. Therefore, in the low flow rate period which is a main driving region under urban driving conditions, the power consumption of the air compressor is increased, and therefore fuel efficiency of the vehicle deteriorates.
In more detail, a pressurizing air compressor having a further increased air compression ratio compared to that of an atmospheric air blower must be configured such that a range of the driving speed of a motor installed thereof is further increased. Accordingly, a difference in the driving speed of the motor between a low flow rate period and a high flow rate period is increased. Therefore, there is a disadvantage in that it is difficult to improve efficiency of the air compressor. That is, in the pressurizing air compressor, as a rotating speed of the motor is increased, a motor inductance is reduced to secure a sufficient voltage margin in a high-speed driving period. Due to the reduction of the motor inductance, three-phase ripple current is increased, and efficiency of the motor/inverter is thus reduced. Particularly, in a low flow rate period in which comparatively low output is required, the three-phase current is small, and efficiency reduction effect is markedly increased due to an increase in a current ripple. That is, three-phase ripple current is a secondary component which does not contribute to the motor torque. Thus, in the low flow rate period having a low motor torque, three-phase ripple current is increased compared to a three-phase sine wave current component, so that the motor/inverter efficiency is reduced compared to that of the high-output period.
Furthermore, to secure high-speed rotation of the motor of the air compressor, an airfoil bearing is used. The airfoil bearing must be rotated at a predetermined speed or more so as to maintain the bearing in a lifted state. Therefore, if the motor is continuously operated at a speed lower than the reference speed over which the airfoil bearing can be maintained in the lifted state, the airfoil bearing may be damaged by friction with a rotating shaft of the motor. Thus, to prevent the airfoil bearing, the air compressor is configured such that the minimum driving speed is limited to a predetermined value or more. Consequently, even when the fuel cell must be operated with low output, the air compressor is operated at speeds higher than the minimum driving speed. Thereby, air is unnecessarily supercharged, thereby reducing efficiency of the entire fuel cell system.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.